A cross arm in a medical X-ray equipment mainly performs the function of supporting a tube and a detector; during use, the cross arm needs to be rotated for taking different X-ray films or images. FIG. 1A shows an X-ray equipment 100 when a cross arm 101 of the medical X-ray equipment 100 is in a horizontal position; FIG. 1B shows the X-ray equipment 100 when the cross arm 101 rotates to a vertical position. When the cross arm 101 is rotated, the distance from a tube 103 to a detector 104 (SID) also needs to be adjusted; since the detector 104 is fixed and cannot move along the cross arm, the adjustment of SID can be realized only by moving the tube 103 along a guide 102 on the cross arm 101. When the tube is moving relatively, the gravity balance of the whole cross arm is broken; here an external force is needed to keep balance of the cross arm, so as to avoid the cross arm from rotating, to keep the center of gravity of the cross arm unchanged during the moving of the tube.
In the prior art, a motor is adopted to control rotation of the cross arm. When SID is adjusted, the motor will bear a higher moment caused by gravity imbalance.
However, for that X-ray equipment in which rotation of a cross arm is manually controlled, the prior art gives no corresponding inspiration regarding how to keep the cross arm balanced when SID is adjusted.